Process for fractionation reboiling

ABSTRACT

A distillation method and apparatus which comprises (a) a distillation zone adapted for the countercurrent contact of liquid and vapor, (b) a reboiler feed liquid compartment (c) means for directing distillation zone liquid into the reboiler liquid feed compartment, (d) a thermosyphon reboiler for partial vaporization of liquid thereto to produce reboiled vapor and liquid flows, (e) a conduit for the flow of liquid from the reboiler liquid feed compartment to the reboiler, (f) a reboiler return liquid compartment, (g) a conduit for the flow of reboiled liquid from the reboiler to the reboiler return compartment, (h) a conduit for the flow of reboiled vapor from the reboiler to the distillation zone, (i) means for communication of liquid by gravity flow between the reboiler feed compartment and the reboiler return compartment to provide substantially equal liquid levels in the two reservoirs, (j) a distillation zone liquid product compartment, (k) means for overflow of liquid from the reboiler return compartment to the product compartment to maintain a substantially constant level of liquid in the reboiler return compartment, and (l) means for withdrawal of liquid product from the product compartment.

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for improving thestability of operation in the thermosyphon reboiling of a fractionationliquid residue by arranging the bottom zone of the fractionator columninto a specific configuration of compartments and baffles.

Distillation processes are widely used in the chemical and petroleumindustries for separation of components of a liquid mixture by partialvaporization of the mixture and separation of the vapor containing themore volatile components from the residue containing the less volatilecomponents. Accordingly, knowledge concerning the design and operationof fractionation columns is possessed by persons skilled in the art andhas been made available in a great many references, including forexample, the Chemical Engineers Handbook (Fourth Edition) published byMcGraw-Hill Book Co., New York, N.Y. Additional prior art is representedby U.S. Pat. No. 2,134,836 illustrating the use of fractionation traysand reboiler systems and U.S. Pat. No. 2,398,213 illustrating afractionation column adapted for reducing thermal decomposition in hightemperature accumulators. U.S Pat. No. 3,766,021 illustrates afractionation column adapted for use with heavy immiscible residues inthe feed system arranged to prevent or limit their contact with thereboiler.

SUMMARY OF THE INVENTION

The invention provides a process for improving the stability ofoperation of a fractional distillation system having an uprightvapor-liquid contacting fractionator, which process comprises:

collecting substantially all of the downflowing liquid from the bottomvapor-liquid contacting means in a reboiler feed compartment,

withdrawing said liquid of said reboiler feed compartment for reboiling,

passing the resulting partially vaporized liquid to a reboiler returnliquid compartment,

equalizing the liquid level in the reboiler feed compartment and thereboiler liquid return compartment via means for liquid communicationbetween said compartments, and

overflowing liquid from the reboiler liquid return compartment to afractionator bottoms product compartment.

The invention further provides an apparatus for fractionating chemicalcompounds which comprises:

(a) an enclosed upright vessel having an upper end and a lower end andan internal volume,

(b) an inlet conduit communicating with said internal volume at a pointintermediate the upper and lower ends of the vessel,

(c) a bottoms fraction outlet conduit communicating with said internalvolume at about the lowermost elevation of the vessel,

(d) a distillate outlet conduit communicating with said internal volumeat the upper end of the vessel,

(e) vapor-liquid contacting means operatively distributed within theinternal volume of said vessel,

(f) a first liquid compartment disposed within said vessel below thebottom vapor-liquid contacting means,

(g) collecting means for passing substantially all of the downflowliquid from the bottom vapor liquid contacting means into said firstliquid compartment,

(h) thermosyphon reboiling means operatively associated with the lowerend of the internal volume of the vessel,

(i) means for removing liquid from said first liquid compartment andpassing said liquid to the reboiler said means comprising a firstconduit connected to the boiler and to the lower portion of said firstliquid compartment,

(j) a second liquid compartment disposed within said vessel at anelevation overlapping that of the first compartment, having means forfluid communication between said first compartment and said secondcompartment said second compartment having liquid overflow means at anelevation intermediate said fluid communication means and the liquidoverflow level of said first compartment, for overflowing liquid fromsaid second compartment into the lower end of the vessel, and

(k) means for passing fluid from the reboiler to the vessel, said meanscomprising a second conduit connected to the reboiler and to the outerwall of the vessel and being disposed to pass liquid to the upper partof said second liquid compartment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents a schematic drawing of a fractionation distillationsystem of the present invention.

FIG. 2 is a cross-sectional view of the column of FIG. 1 along line"2--2" and presents the view seen looking downward toward the bottomimmediately above the reboiler return.

FIG. 3 is a schematic drawing of the lower portion of another embodimentof the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The distillation process and apparatus according to the invention isapplicable to the distillation and fractionation of any distillable orfractionable material such as, for example, animal, mineral or vegetableoils and distillate organic and inorganic chemical mixtures orsolutions. It is well suited for fractionation of hydrocarbon oils asmay be obtained from crude petroleum, or synthesized from e.g., carbonoxides and hydrogen, and is particularly well suited for thermallyunstable fractionable materials such as olefins since it features noreboiler bypassing enabling lower reboiler temperatures compared toconventional design.

In the subject invention the feed stream enters the upright, i.e.vertically aligned fractionation column at an elevation above thecompartments hereinafter described and is above collecting means todirect substantially all downflowing liquid into a first reboiler feedcompartment. The column is reboiled in a customary manner through theuse of an internal or external reboiler. Heat is removed also in aconventional manner at the top of the column e.g., by a stabbed-incondenser or an external condenser in an overhead vapor line. The columnis maintained at fractionation conditions selected to separate the feedstream into at least one distillate (overhead) product stream and aresidual (bottoms) stream of different composition. Where desired thecolumn may contain a plurality of vapor liquid contacting materialsand/or structures such as single pass or multi-pass fractionation platesor trays, and a plurality of side draws for withdrawing other desireddistillate fractions of the feed to the column.

When the fractionation system according to the invention is employed asa fractionating column, typically there will be vapor liquid contactingmeans disposed in the column both above and below the feed inlet andcondensing means operatively associated with the upper part of thecolumn. In an embodiment for service as a reboiled stripper, typicallyvapor liquid contacting means will be disposed in the column only belowthe feed inlet, and usually condensing means will be omitted. In anembodiment for service as a reboiled absorber typically there would beno vapor liquid contacting means disposed in the column below the feedinlet, but only collecting means for directing feed liquid to the first(reboiler feed) compartment, such collecting means as e.g., a chimneytray; condensing means would usually be omitted.

A broad range of fractionation conditions includes a pressure of fromsubatmospheric to about 180 bar and a bottoms temperature of from about-100° C. to about +350° C. Preferred fractionation conditions include apressure in the range from about 0.5 to 35 bar and a bottoms temperaturein the range from about 100° to about 280° C. The reflux ratiopreferably is in the range from about 0.1:1 to about 10:1. Otherconditions such as vapor velocity, liquid loadings and tray designs maybe selected by those skilled in the art for the particular fractionationapplication.

A preferred embodiment of the invention is shown in FIG. 1. A feedstream such as gas oil fraction from a hydrocarbon cracking unit entersa vertically aligned trayed fractionation column 2 through line 1. Thiscolumn is maintained at conditions required for fractionation of thefeed stream by vaporization in reboiler 3 of liquid from bottom tray 4passed via downcomer means 5 and liquid seal pan 17 into liquid(reboiler feed) compartment 6 and carried to the reboiler by conduit 7.Downcomer means 5 may have a conventional seal pan as shown or mayextend below the liquid level in compartment 6. As will be appreciatedby those skilled in the art, conduit 7 could exit the sidewall of thevessel instead of the bottom, if desired, for example to minimize pipingto a short reboiler. The at least partially vaporized liquid fromreboiler 3 is returned to the column via conduit 8. Optional deflectorplate 9 directs the liquid portion of the reboiler return stream towardbaffle means 10 which carries the liquid to a second liquid (reboilerreturn) compartment 11. Baffle means 10, which may be level or sloping,is disposed to also direct any weepage from that portion of the bottomtray 4 which is directly above the baffle into the reboiler returncompartment. A major portion of the return liquid overflows the returncompartment 11 into the bottoms product compartment 12. The reboilerreturn compartment will normally be full and overflowing. Communicationmeans 13, disposed above the lowermost elevation up to about the upperlevel in the return compartment, provides a path for return liquid tothe reboiler feed compartment 6 and assures that levels are constant andessentially the same in both compartments. Such communication means maycomprise an aperture but preferably is a horizontal slot in the commonwall when the reboiler feed compartment 6 and reboiler returncompartments are adjacent as shown, or when the compartments areseparated as shown in FIG. 3 may comprise a conduit.

During a typical operation where the flow of liquid from bottom tray 4is greater than flow to the reboiler, liquid will flow throughcommunication means 13 from the reboiler feed compartment 6 into returncompartment 11. Constant level in both compartments is still maintainedby the liquid overflowing the return compartment 11 to the bottomsproduct compartment 12.

In a preferred embodiment communication means 13 is provided with atleast one baffle means such as trap 14 to minimize convection flowbetween the two compartments (the liquids are usually of slightlydifferent density). Also desirable is an optional deflection baffle 15above communication means 13 to reduce or prevent disturbance to flow incommunication means from entry of the downcoming fluid from bottom tray4.

In a particularly preferred embodiment the reboiler return compartmenthas one or more small apertures in the bottom of the compartment to passa minor portion of the liquid and purge any solid particles such asscale, degradation products and the like into product compartment 12. Ineffect then the zone in the reboiler return compartment 11 which isbelow the elevation of communication means 13 is a settling zone. Thecombination of settling zone and purging of dense material and solidparticulates removes such materials from the reboiler loop. Otherwise,corrosion products, silt introduced with column feed, fouling depositsand the like would tend to be trapped in the reboiler feed and returnloop, which, in many cases would lead to increased reboiler fouling.

The top of the fractionating tower operates in a conventional manner.Vapors containing the lower boiling components of the feed stream risethrough a plurality of fractionation trays 18 and are removed as anoverhead vapor stream carried by line 20. This vapor stream is passedthrough a condenser 22 and then directed via line 24 into an overheadreceiver 26. Uncondensed vapors, if any, are removed by a line notshown. The condensed overhead liquid is drained in line 28 and dividedinto a reflux stream carried by line 30 and a net overhead productstream removed in line 32.

The higher boiling components of the feed which have collected in thelower end of the fractionation column in bottoms product compartment 12are withdrawn by conduit 26 located at about the lowermost elevation ofthe vessel.

FIG. 2 presents a cross-section of the column 2 of FIG. 1 taken alongsection line 2--2 looking downward at reboiler feed compartment 6,reboiler return compartment 11 and baffle collection means 10.Downcoming liquid from bottom tray 4 is collected in compartment 6, andpassed by conduit 7 to reboiler 3 and returned to column 2 by conduit 8.The unvaporized portion of the returned liquid is collected and directedby baffle means 10 into return compartment 11. A small portion of thereturned liquid is purged into the underlying product compartment, notshown, via apertures 16 in the bottom of the return compartment. Liquidmay pass through fluid communication means not shown to maintainsubstantially common liquid level in the reboiler feed compartment andthe reboiler return compartment. Substantially, constant level ismaintained by liquid overflowing the reboiler return compartment intothe bottom product compartment.

FIG. 3 represents a different mechanical embodiment of the invention.FIG. 3 is a cross-sectional view of a portion of a packed column inwhich the same numbers designate elements common with those in FIGS. 1and 2. In this embodiment, the vapor-liquid contacting means comprises acommercially available packing material 34 operatively disposed withinthe inner volume of the fractionation vessel 2, at points above andbelow the inlet conduit. An upper bed of packing is held above the feedinlet 1 by a horizontal screen 36 which conforms generally to the innersurface of the vessel. A lower bed of packing is positioned below thefeed inlet 1 and held above the lower end of the vessel by a similarhorizontal screen 38. The feed enters the column and decends through thepacking counter current to rising vapors. Liquid descending throughscreen falls either directly into reboiler feed compartment 6 or uponbaffle means 40 for passing substantially all of downflow liquid fallingupon it into said reboiler feed compartment.

The size of the reboiler feed and return compartments will depend uponthe specific application. In general, the superficial vertical velocityin each compartment should be less than 10 centimeters per second andpreferably less than 5 centimeters per second. The height of liquid inthe compartments above the fluid communication means 13 is preferablygreat enough to suppress vaporization upon mixing of the two liquidswhen there is flow of liquid from the reboiler return compartment 11into reboiler feed compartment 6. The particular arrangement best suitedto a given application will be determined in large measure by the lengthof the reboiler and the amount of surge volume required in the bottomsproduct compartment 12.

The vessel arrangement below the vapor liquid contacting means in FIG. 1applies for long reboilers and relatively high bottoms product surgevolume; in FIG. 3 for relatively small bottoms product surge regardlessof reboiler size.

As will be appreciated by those skilled in the art, there are manyadvantages according to the invention. Some of these include: that theliquid descending from the bottom vapor-liquid contacting means ispreferably fed to the reboiler enabling the use of the lowest reboilertemperature and highest degree of staging in the reboiler for a givenreboiler feed rate; the relatively constant level in the reboiler feedcompartment stabilizes the circulation rate through the reboiler loopfor thermosyphon reboilers; and in trayed columns virtually all trayweepage is caught and potentially can reach the reboiler, enabling thefractionation column to operate at a very low fraction of normal load,i.e. a very high turndown ratio, regardless of the design or conditionof the bottom tray. This feature avoids the requirement for specialstart procedures often required with thermosyphon reboilers. Furtheradvantages include: that particulates are continuously purged from thereboiler circulation loop; the length of piping in reboiler loop isreduced; in the arrangement of FIG. 1 the height of the bottoms productscompartment may be reduced for a given bottoms surge volume enabling areduction in overall column height; and multiple reboilers (not shown)can easily be accommodated with no loss in these advantages. Finally,the invention is especially advantageous for fractionation of thermallyunstable materials since the features of no reboiler bypassing, lowerreboiler temperatures and purging of particulate and decompositionproducts from the reboiler circulation loop result in improvedperformance compared to conventional designs.

The following illustrative embodiment, based upon calculated resultswill serve to provide a fuller understanding of the invention, howeveris not to be interpreted as limitative of the invention. A conventionalfractionation distillation tower equipped with an external thermosyphonreboiler is used to "split" a full boiling range naphtha fraction into alight naphtha fraction substantially boiling at atmospheric pressure attemperatures less than 93° C. and a less volatile heavy naphtha fractionsubstantially boiling at atmospheric pressure at temperatures above 93°C. The column is operated at 1.72 bar at a feed rate of 44,000 Barrelsper day (B/D). The column is operated with 20,500 B/D reflux and resultsin the separation of 17,000 B/D of light naphtha as overhead; 27,000 B/Dof heavy naphtha is removed as bottoms at a temperature of about 143° C.The feed to the thermosyphon reboiler is taken from the surge volumenear the bottom of the column at 143° C. through the reboiler having areboiler duty of forty million BTU's per hour, and is returned to thecolumn above the surge volume at a temperature of about 150° C. Thisoutlet temperature will vary somewhat as the liquid level in the surgevolume of the column fluctuates.

When the above column is equipped with the system of compartments andconduits according to the invention as shown in FIG. 1, it is found thatthe temperatures of the feed to the reboiler is 135° C. and thetemperature of the effluent from the reboiler is 143° C., i.e. 7° C.lower. Moreover the constant level in the reboiler feed compartmentsteadies the temperatures in the column, permitting stable operationirrespective of fluctuations in the surge volume.

I claim:
 1. A process for improving the stability of operation of afractional distillation system having an upright vapor-liquid contactingfractionator, which process comprises:collecting substantially all of adown flowing liquid from a bottom vapor-liquid contacting means in areboiler feed compartment, withdrawing liquid exclusively from saidreboiler feed compartment for reboiling, passing said liquid to areboiler, wherein said liquid is partially vaporized, passing theresulting partially vaporized liquid from the reboiler to a separatereboiler return liquid compartment, equalizing the liquid level in thereboiler feed compartment and the reboiler liquid return compartment viameans for liquid communication between said compartments said meansbeing disposed above the bottom of said reboiler liquid returncompartment, thereby forming a settling zone within said liquid returncompartment, and overflowing liquid from the reboiler liquid returncompartment to a lower fractionator bottoms product compartment.
 2. Aprocess as in claim 1 comprising the additional step of passing a minorportion of the liquid in the reboiler liquid return compartment from thereboiler liquid return compartment via at least one aperture in thebottom of said compartment into the fractionation bottoms productcompartment for purging any particulates from said liquid returncompartment.